They are very different. Nat had a bit of a pure, crooner voice whereas Porter has a really rich, soulful voice. I personally prefer Porter's singing, but Nat King Cole was a great pianist which is often forgotten.
Gregory Porter singing ‘it’s probably me’ the theme to Lethal Weapon and written by Sting, is a delight to hear.
But I’ll always give it to Nat King Cole
Gregory Porter , he can change from jazz to dance with ease, holding on with disclosure is one of my all time favourites , and Concorde was the last song I sang at a karaoke, what a song